Traditional machining methods, which are the principal means of removing metal from workpieces, include chip cutting (such as milling, drilling, turning, broaching, reaming, and tapping) and abrasive machining methods (such as sanding, grinding, and polishing. One such chip cutting process, face milling, may be useful to produce a generally flat surface on a workpiece. A face milling tool or “face mill” is so named because the flat workpiece surface is produced by action of the face of the tool, although the outside diameter or bevel cutting edge removes most of the stock. In a typical application, a milling cutter tool comprising a number of cutting inserts may be driven by a spindle on an axis positioned perpendicular to the surface being milled. ASM Handbook, Volume 16, “Machining” (ASM Intern. 1989) p. 311.
A milling cutter tool produces chips with variable chip thickness. Chip thickness may be used in calculating the maximum load per unit length exerted on the edges of a milling cutting tool. An average chip thickness is typically used in such calculations. Average chip thickness can be calculated and varies with cutting insert lead angle for the same material feed rate. For the example of a substantially square-shaped insert having four identical cutting edges, a larger lead angle produces a larger average chip thickness during machining, while a smaller lead angle produces chips of smaller average thickness. An example of the variation of average chip thickness with lead angle of the insert is shown in FIG. 1. FIG. 1 illustrates a comparison of an identical square-shaped insert machining with lead of angles of 90°, 75°, and 45°. As indicated in the FIG. 1, as the lead angle increases from 45° in FIG. 1(a), to 75° in FIG. 1(b), to 90° in FIG. 1(c), the average chip thickness (hm) increases from 0.71 times the feed per tooth of the holder (“fz”), to 0.97×(fz), to fz. More generally, the chip thickness for a square-shaped cutting insert, or any other insert having a linear cutting edge used in a milling cutter tool, may be calculated using the equation hm=fz× sin (K), where hm is the average chip thickness, and K is the lead angle measured in the manner shown in FIG. 1.
FIG. 1 also indicates that the length of engaged cutting edge when using a 90° lead angle is shortest among those scenarios shown in FIG. 1, while the length of engaged cutting edge is longest when the lead angle is 45°. This means that face milling using a 90° lead angle produces more load, i.e., higher stresses, on the cutting edge per unit length compared with milling using a 45° lead angle, for the same depth of cut. An advantage of reducing load on the cutting edge per unit length is that reduced load allows for employing a higher feed rate per tooth in the milling operation and improved tool life. Thus, to reduce the average load stresses on the engaged cutting edge, it is clearly an advantage to use a smaller lead angle.
Square-shaped cutting inserts are commonly used in face and plunge milling because they are strong, indexable and have multiple cutting edges. Inserts having a substantially square shape or otherwise including four cutting edges are disclosed in, for example, U.S. Pat. Nos. 5,951,212 and 5,454,670, U.S. Published Application No. US2002/0098049, Japanese reference No. 08174327, and PCT Publication No. WO96/35538. A common feature of the inserts disclosed in these references is the combination of four straight cutting edges and either a planar or a bevel planar clearance (or relief) surface below each cutting edge.
It is well-known that round-shape inserts, however, have the strongest cutting edge. In addition, round-shaped inserts provide a favorable combination of maximal corner strength, good material removal capacity, mechanical shock resistance, and thermal distribution. As such, round-shaped face milling inserts are often used for the more demanding machining applications, such as those involving difficult-to-cut materials, hard materials, heat resistant materials, titanium, etc. In face milling using a round-shaped cutting insert, the lead angle and the extent of the engaged cutting edge will vary with the depth of cut, as shown in FIG. 2. The average chip thickness produced by a round-shape insert can be approximately calculated by the following equation (I):
                              h          m                =                                            f              z                        R                    ·                                                    R                2                            -                                                (                                      R                    -                    doc                                    )                                2                                                                        (        I        )            
where hm is the average chip thickness, fz is the feed per tooth from a milling cutter, R is the radius of the round-shape cutting insert, and doc is the depth of cut. The above equation indicates that when cutting with a round-shaped insert, chip thickness varies with depth of cut. In contrast, when cutting using a square-shaped insert or any insert having a linear cutting edge, chip thickness does not change with changes in the depth of cut if the lead angle remains the same (see FIG. 1).
Furthermore, for the same depth of cut, a larger radius of a round-shaped insert always corresponds to a larger portion of the cutting edge engaging the work piece, as illustrated in FIG. 3, thus, reducing the average stress load per unit length on the cutting edge. This, in turn, allows the use of higher feed rates during face milling without a loss of quality. However, a limitation of a round-shaped cutting insert lies in that the larger the radius, the larger the insert. It is difficult to fully utilize the advantages provided by round-shaped inserts of increasingly larger radius in conventional machining applications due to their size.
Accordingly, to overcome the cutting edge load problems that may be encountered in face milling with large lead angles, there is a need for an improved design of cutting insert that allows for significantly increased feed rates during face milling operations while maintaining the same or longer tool life of the cutting inserts. Also, there is a need for a new cutting insert that is similar to a round-shaped insert in that it exhibits favorable cutting edge strength, but also is similar to a square-shaped insert in that it includes multiple cutting edges, is indexable, and also allows for a high feed rate and favorable wear properties.